The present invention is related to the field of nuclear magnetic resonance (xe2x80x9cNMRxe2x80x9d) sensing apparatus. More specifically, the invention is related to a NMR well logging apparatus having a powdered soft magnetic material core as a flux concentrator for sensing NMR properties within earth formations adjacent a wellbore.
NMR well logging instruments can be utilized for determining properties of earth formations including: the fractional volume of pore space, the fractional volume of mobile fluid filling the pore space and other petrophysical parameters. An NMR well logging instrument typically contains a permanent magnet to generate a static magnetic field in adjacent earth formations. The NMR well logging instrument typically includes a transmitting antenna assembly positioned near the magnet. The transmitting antenna assembly is shaped so that a pulse of radio frequency (RF) power conducted through the antenna assembly induces a RF magnetic field in the adjacent earth formation. The induced RF magnetic field is generally orthogonal to the static magnetic field, thereby creating appropriate conditions for NMR excitation in the formation.
Following the RF antenna pulse, voltages representative of NMR conditions in the formation are induced in the receiving antenna. In particular, these voltages represent precessional rotation of hydrogen or other nuclei spin axes about the static magnetic field generated by the NMR well logging tool. NMR apparatus designs typically use the same antenna for transmitting and receiving along with de-coupling, receiving and protection circuitry.
There are various known NMR well logging instruments proposed and/or implemented for measuring NMR properties of substances, in particular, the properties of earth formations. One type of NMR instrument is described in U.S. Pat. No. 4,710,713 (xe2x80x2713), by Taicher et al. Another type of NMR instrument is described in U.S. Pat. No. 4,350,955 (xe2x80x2955), by Jackson et al. Both of these NMR instruments represent early designs of well logging NMR instruments with the main focus on the magnet assembly. No provision was made in these early designs for the use of a soft magnetic material in the NMR probe for improving the efficiency of the RF antenna.
It was recognized in more recent NMR well logging tool designs that a soft magnetic material can be utilized as a magnetic flux concentrator to increase efficiency of generating and receiving RF signals. For example, the NMR well logging instruments using ferrite material as an essential element of the design are described in U.S. Pat. No. 5,712,566 (""566), by Taicher et al., in U.S. Pat. No. 5,644,231(""231) by A. H. Wignall, in UK Patent Application GB 2 141 236 (""236) by A to H. Clow, et al. and in U.S. Pat. No. 5,376,884 (""884) by A. Sezginer.
All prior designs known to the present inventors, however, explicitly or implicitly suggest ferrite as the soft magnetic material satisfying the requirements of high permeability and negligible RF losses. Ferrite materials, however, suffer from a relatively low saturation flux density, typically in the range of 0.3-0.4 T. This relatively low saturation flux density results in core saturation when the ferrite core is placed near the NMR probe permanent magnet. NMR probe core saturation results in reduction of the core magnetic permeability which tends increase core sensitivity to temperature variations. A sintered ferrite material core tends to generate magnetostrictive ringing in a strong static or RF magnetic field. Elimination of this parasitic magnetostrictive ringing signal increases the complexity and cost of NMR antenna design.
A common limitation of the ""231, ""566, ""884, and ""236 patent designs is the necessity of finding or creating a substantially zero magnetic field in a region where the soft ferrite material can be positioned to avoid saturation. For example, the apparatus disclosed in the ""231 patent provides a soft magnetic ferrite material loaded in the antenna coil (a so called half-coax antenna). As described in the ""231 patent, the effectiveness of the ferrite material is substantially reduced by the strong magnetic field of the permanent magnet. The structure of the ""231 patent compensates for this reduction in effectiveness by providing a magnetic shield around the ferrite region. The shield comprises a shell of soft magnetic steel, which effectively provides a shunt path for static magnetic field in the region of the antenna. Implicitly, the steel shell is not saturated due to its sufficient saturation flux density and cross-sectional area. The necessity of creating a region of substantially zero static magnetic field places a serious constraint on the design of NMR probes. In particular it places limitations on the antenna core size, thereby reducing the efficiency of the antenna. Thus only a region very close to such a NMR antenna can be effectively analyzed.
Thus, there is a need for a NMR probe core material that overcomes the limitations of prior art discussed above.
The present invention provides a novel use of a powdered high saturation flux density soft magnetic material as a NMR probe core material. The probe structural geometry facilitates the use of powdered material, which has a relatively low magnetic permeability. In accordance with a preferred embodiment of the present invention a nuclear magnetic resonance sensing apparatus is provided, comprising a magnet for inducing a static magnetic field in materials to be analyzed; an antenna assembly for inducing a radio frequency magnetic field within the materials and for detecting nuclear magnetic resonance signals from the materials, the antenna comprising at least one magnetic core formed from a powdered soft magnetic material having high saturation flux density and a non-conductive bonding agent.
The magnetic core has dimensions related to the direction of RF magnetic field and to magnetic permeability of the powdered soft material. In particular, an effective demagnetizing factor of the magnetic core in the direction of the radio frequency magnetic field substantially exceeds the inverse magnetic permeability of the powdered soft magnetic material. As applied to NMR oil-well logging the present invention provides a permanent magnet and an antenna elongated in the direction of bore-hole, the permanent magnet and the antenna assembly disposed adjacent one another. The dipole magnetic moments of the antenna and the magnet are perpendicular to one another the direction of elongation. A variety of embodiments of this type of structure are presented.
There are numerous advantages associated with use of the preferred powdered soft magnetic core material and NMR probe structure of the present invention. The NMR powdered soft magnetic probe core material and probe structure provided by the present invention enables optimization of RF antenna efficiency in NMR probes without incurring the practical limitations of ferrite NMR probes. Ferrite NMR probe cores are less efficient than the preferred probe of the present invention, due to potential saturation of the ferrite by the static magnetic field of NMR probe permanent magnets. The core material of the present invention is not saturated by the NMR probe magnetic field because of the high saturation flux density of the preferred core material. Therefore, the preferred core material can be placed close to a strong permanent magnet in a NMR probe without saturating the soft magnetic material and diminishing the efficiency of the RF antenna in the NMR probe.
In a preferred embodiment, the RF magnetic flux is concentrated in the preferred core, thus, the conductivity of the probe permanent magnet does not reduce RF antenna efficiency of the probe, thereby enabling utilization of the strongest available commercial magnets. The preferred powdered core material reduces or eliminates magnetostrictive ringing by virtue of the particulate structure of the preferred material. The magnetic particle size of the preferred core material (powder) is substantially smaller than the minimum wavelength for acoustic excitation associated with magnetostrictive ringing. Moreover, the preferred probe antenna core magnetic and electrical characteristics are more stable than ferrite core characteristics in the presence of temperature variations.
Further features and advantages of the invention will become more readily apparent from the following detailed description, when taken in conjunction with the accompanying drawings.
The application is best understood with reference to the following drawings wherein like numbers in different figures refer to like components.